1. Field of the Invention:
This invention relates to a method for the production of a fiber-reinforced thermosetting resin molding material. More particularly, this invention relates to a method for the production of a fiber-reinforced thermosetting resin molding material having a liquid thermosetting resin composition and reinforcing fibers dispersed in and mixed with each other without exertion of any immoderate force and having the reinforcing fibers satisfactorily impregnated with the thermosetting resin composition without infliction of any breakage.
2. Description of the Prior Art:
Generally, the fiber-reinforced thermosetting resin molding material for hot pressing or injecting is obtained by combining the thermosetting resin liquid such as unsaturated polyester resin with additives such as fillers, mold release agents, thickening agents, and coloring agents thereby producing a viscous liquid thermosetting resin composition and mixing this resin composition with reinforcing fibers such as glass fibers thereby enabling the fibers to be impregnated with the resin composition. At the time of molding, therefore, the molding material so produced is tack free enough to ensure ease of handling. Heretofore, the sheet molding compound (SMC) supplied in the form of sheet and the bulk molding compound (BMC) suplied in bulk have been mainly used. As compared with the SMC process which causes a liquid composition of relatively low viscosity and reinforcing fibers to be mixed with each other and thereby enabling the reinforcing fibers to be impregnated with the resin composition, the BMC process generally involves impregnation with a resin composition incorporating a large amount of filler and exhibiting high viscosity and, therefore, has no alternative but to use a powerful stirrer such as a kneader in attaining satisfactory mixture of the liquid resin composition with the reinforcing fibers and consequent impregnation of the reinforcing fibers with the resin composition.
In accordance with the conventional BMC process, the staple cutting length of reinforcing fibers for use in the molding material is short, falling on the order of 1/2 inch to 1/4 inch. Moreover, this process has the disadvantage that the reinforcing fibers, while being kneaded with the resin composition, eventually reach the point of sustaining breakage under the shearing force generated because of the stir by a kneader and the shaped article produced by using the consequently obtained molding material suffers from insufficient strength. As the molding material of required consistency is produced in consequence of the stirring by the kneader, it is removed from the stirring tank by the use of an extruder, wide difference of strength occurs between the portion of the material removed during the first half of the whole duration of removal of one lot of produced material and the portion removed during the latter half of the duration. From the standpoint of quality control, therefore, the conventional BMC process possesses the drawback that the molding material obtained in one lot has its strength heavily dispersed from one part to another.
In the conventional SMC process, reinforcing fibers are spread between layers of liquid resin composition applied one each on two opposed polyethylene films and the layer of the reinforcing fibers as nipped between the layers of liquid resin composition is pressed with a roller so as to cause impregnation of the reinforcing fibers with the resin composition. The sheet molding material produced by any method of this principle, therefore, has a thickness generally on the order of 2 to 3 mm at most. By simply increasing the amount of the resin composition to be applied and the amount of the fibers to be spread, the sheet molding material of increased thickness cannot be obtained with high productivity.
As a means of enabling reinforcing fibers to be most easily mixed and impregnated with a resin composition without entailing infliction of breakage, the method which comprises applying the resin composition on a pair of rollers and nipping a layer of chopped glass fibers between the aforementioned rollers thereby causing the glass fibers to be impregnated with the resin composition by virtue of the pressure exerted by the opposed rollers upon the layer of glass fibers has been known to the art (U.S. Pat. No. 3,932,980). This method is basically identical with the SMC process because it causes the glass fibers to be impregnated with the resin composition of high viscosity by virtue of the pressure of the opposed rollers. Accordingly this method enables the impregnation to proceed only to a limited extent. Particularly when the resin composition so used has high viscosity and the glass fibers' content in the produced molding material is high, it is extremely difficult for the resin composition to provide proper impregnation of the glass fibers. Further the impregnation is not obtained sufficiently because it is instantaneously effected solely by the linear pressure generated while the resin composition and the glass fibers are passing between the opposed rollers. The glass fibers are injured because they are exposed instantaneously to the excessive force. The mixture of glass fibers and resin composition emerging from the pair of nip rollers is scraped off these nip rollers with a rotary roller. Since this rotary roller is operated at a high speed, the glass fibers in the scrapped mixture are liable to be disentangled and the shaped article produced by using the mixture is liable to suffer from degradation of strength. A more important thing is the fact that this method has low productivity. The combined amount of the resin composition and the glass fibers to be passed between the paired nip rollers per unit time and the speed of their passage have their limites. When the rotational speed of the rotary rollers is increased, the impregnation is not obtained to a sufficient extent. This method, therefore, has the drawback that it is difficult to increase productivity.
As a means of solving problems of this nature, the spray method is known which enables chopped glass fibers to be mixed and blended most effectively with a resin composition. In accordance with the spray method, however, the highest viscosity the resin composition is tolerated to possess in order to be effectively divided into fine particles by spraying is only on the order of 20 to 30 poises. The highest viscosity the resin composition is tolerated to possess to be effectively divided into coarse particles by spraying is only on the order of 100 poises.
In contrast, the liquid resin composition generally used for BMC has viscosity in the range of 1,000 to 10,000 poises. Thus, the resin composition cannot be supplied by the spray method to the glass fibers.
As the result of studies for the solution of the problems involved in the conventional BMC method, SMC method, or spray method as described above, we have found the fact that a fiber-reinforced molding material of thermosetting resin possessing usually unattainable excellent impact strength is efficiently obtained by spreading a liquid thermosetting resin composition of high viscosity in the form of particles and allowing these particles of resin composition to be mixed with separately spread reinforcing glass fibers without exertion of immoderate force owing to the use of two kinds of rollers. The invention materializing this finding has been already filed for patent (Japanese Patent Laid-Open No. SHO 60(1985)-97,808 and U.S. Pat. No. 4,702,872).
In the method of the patent application mentioned above, the fact that the two kinds of rollers, i.e. transferring (primary) rollers operated at a high speed in the range of 200 to 1,000 r.p.m. and spreading (secondary) rollers disposed at a slight distance from the transferring rollers and operated at a higher speed in the range of 3,000 to 10,000 r.p.m., are to be installed appreciably adds to the implication and size of the mechanism of spreading devices and increases the cost of equipment. An idea .of designing the spreading (second) rollers for high-speed rotation in a detachable form and adapting the two kinds of rollers so as to be separated with a controlled gap is impracticable because these rollers are operated at high speeds. From the operational point of view, since the two kinds of rollers are operated as juxtaposed to each other, this method has a disadvantage that the rollers cannot be easily cleaned after use.
In the method mentioned above, the resin composition supplied onto the transferring (primary) rollers is spread out in the form of particles by the spreading (secondary) rollers rotated at a high speed as separated by a gap from the transferring (primary) rollers. Part of the resin composition so supplied onto the transferring (primary) rollers adheres persistently on the surface of the transferring (primary) rollers because of the viscosity. The adhering resin composition constitutes itself a cause for impairing the uniform dispersion and mixture of the resin composition and the reinforcing fibers. When the part of the resin composition thus adhering to the transferring (primary) rollers continues to remains completely irreplaceable with the freshly supplied resin composition, the adhering resin composition on the surface of the transferring (primary) rollers gains in viscosity. With elapse of time, the adhering resin composition of increased viscosity forms a layer on the surface of the rollers. Pieces of viscous resin composition which subsequently peel off the surface of the rollers are suffered to intervene between superposed piles of separately spread reinforcing fibers and interfere with desired uniform mixture and impregnation of the reinforcing fibers with the resin composition, to pose a problem.
An object of this invention, therefore, is to provide a novel method for the production of a fiber-reinforced molding .material of thermosetting resin.
Another object of this invention is to provide a method for the production of a fiber-reinforced molding material of thermosetting resin, which method enables a liquid thermosetting resin composition and reinforcing fibers to be uniformly dispersed and mixed with each other, avoids inducing breakage of the reinforcing fibers, and exhibits very high productivity.
Still another object of this invention is to provide a method for the production of a fiber-reinforced molding material of thermosetting resin, which method permits an apparatus for the production of the molding material, especially a device for the spreading of resin composition, to be handled with high operational efficiency and stably attains the uniform dispersion and mixture of the resin composition and the reinforcing fibers without requiring the apparatus to be given any protracted maintenance.